# std::basic_string<CharT,Traits,Allocator>::find

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< cpp‎ | string‎ | basic string

size_type find( const basic_string& str, size_type pos = 0 ) const; | (1) | (noexcept since C++11) (constexpr since C++20)
size_type find( const CharT* s, size_type pos, size_type count ) const; | (2) | (constexpr since C++20)
size_type find( const CharT* s, size_type pos = 0 ) const; | (3) | (constexpr since C++20)
size_type find( CharT ch, size_type pos = 0 ) const; | (4) | (noexcept since C++11) (constexpr since C++20)
template< class StringViewLike > size_type find( const StringViewLike& t, size_type pos = 0 ) const noexcept(/* see below */); | (5) | (since C++17) (constexpr since C++20)

Finds the first substring equal to the given character sequence. Search begins at pos, i.e. the found substring must not begin in a position preceding pos.

1) Finds the first substring equal to str.
2) Finds the first substring equal to the range `[`s`,` s + count`)`. This range may contain null characters.
If `[`s`,` s + count`)` is not a valid range, the behavior is undefined.
3) Finds the first substring equal to the character string pointed to by s. The length of the string is determined by the first null character using Traits::length(s).
If `[`s`,` s + Traits::length(s)`)` is not a valid range, the behavior is undefined.
4) Finds the first character ch (treated as a single-character substring by the formal rules below).
5) Implicitly converts t to a string view sv as if by std::basic_string_view<CharT, Traits> sv = t;, then finds the first substring equal to sv.
This overload participates in overload resolution only if std::is_convertible_v<const StringViewLike&,
std::basic_string_view<CharT, Traits>> is true and std::is_convertible_v<const StringViewLike&, const CharT*> is false.

Formally, a substring str is said to be found at position xpos if all of the following are true:

- xpos >= pos
- xpos + str.size() <= size()
- for all positions n in str, Traits::eq(at(xpos + n), str.at(n)).

In particular, this implies that

- a substring can be found only if pos <= size() - str.size()
- an empty substring is found at pos if and only if pos <= size()
- for a non-empty substring, if pos >= size(), the function always returns npos.

### Parameters

str | - | string to search for
pos | - | position at which to start the search
count | - | length of substring to search for
s | - | pointer to a character string to search for
ch | - | character to search for
t | - | object (convertible to std::basic_string_view) to search for

### Return value

Position of the first character of the found substring or npos if no such substring is found.

### Exceptions

1,4) Throws nothing.
5)
`noexcept` specification:
noexcept(std::is_nothrow_convertible_v<const T&, std::basic_string_view<CharT, Traits>>)

If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).

### Example

Run this code

    #include <iomanip>
    #include <iostream>
    #include <string>

    void print(int id, std::string::size_type n, std::string const& s)
    {
        std::cout << id << ") ";
        if (std::string::npos == n)
            std::cout << "not found! n == npos\n";
        else
            std::cout << "found @ n = " << n << ", substr(" << n << ") = "
                      << std::quoted(s.substr(n)) << '\n';
    }

    int main()
    {
        std::string::size_type n;
        std::string const s = "This is a string"; /*
                                 ^  ^  ^
                                 1  2  3          */

        // search from beginning of string
        n = s.find("is");
        print(1, n, s);

        // search from position 5
        n = s.find("is", 5);
        print(2, n, s);

        // find a single character
        n = s.find('a');
        print(3, n, s);

        // find a single character
        n = s.find('q');
        print(4, n, s);
    }

Output:

    1) found @ n = 2, substr(2) = "is is a string"
    2) found @ n = 5, substr(5) = "is a string"
    3) found @ n = 8, substr(8) = "a string"
    4) not found! n == npos

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 847 | C++98 | there was no exception safety guarantee | added strong exception safety guarantee
LWG 2064 | C++11 | overloads (3,4) were noexcept | removed
LWG 2946 | C++17 | overload (5) caused ambiguity in some cases | avoided by making it a template
P1148R0 | C++11 C++17 | noexcept for overloads (4,5) were accidently dropped by LWG2064/LWG2946 | restored

### See also

strstr | finds the first occurrence of a substring of characters (function)
wcsstr | finds the first occurrence of a wide string within another wide string (function)
strchr | finds the first occurrence of a character (function)
wcschr | finds the first occurrence of a wide character in a wide string (function)
rfind | find the last occurrence of a substring (public member function)
find_first_of | find first occurrence of characters (public member function)
find_first_not_of | find first absence of characters (public member function)
find_last_of | find last occurrence of characters (public member function)
find_last_not_of | find last absence of characters (public member function)
find | find characters in the view (public member function of `std::basic_string_view<CharT,Traits>`)
search | searches for the first occurrence of a range of elements (function template)

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